Direct injection gasoline engines offer improved efficiency because fuel injected directly into a cylinder can reduce cylinder charge temperature. As a result, additional air may enter a cylinder as compared to an equivalent cylinder that has port injected fuel. Consequently, engine power and efficiency may be improved. In addition, direct injection gasoline engines may exhibit improved transient fuel control because there is less tendency for fuel to collect at a cylinder intake port of a direct injection engine than for a port fuel injection engine. However, direct injection engines may generate soot at higher engine speed and load conditions because there is less time available to atomize fuel in the cylinder. As a result, it may be useful to incorporate a particulate filter in the exhaust system of a direct injection engine. Gasoline engines include those engines fueled by pure gasoline or mixtures of gasoline and other fuels such as alcohols. Further, other fuels used in spark ignited engines are also included such as liquid propane gas (LPG) or compressed natural gas (CNG).
In U.S. Patent Application 2009/0193796 a system for treating exhaust gases of a gasoline engine is presented. In several embodiments, a three-way catalyst is followed by a particulate filter. The particulate filter may be coated with various combinations of platinum, palladium, and rhodium. The coated particulate filter may assist in the oxidation of soot that is held by the particulate filter. It may be beneficial to filter gasoline engine emissions with a particulate filter, but over time, a particulate filter may accumulate an amount of soot to the extent that it reduces engine efficiency by increasing backpressure in the exhaust system. The reference appears to provide little direction as to how to remove soot from a particulate filter. Therefore, the system described in the reference may cause engine performance to degrade over time. In addition, the three-way catalysts described in the reference operate at higher efficiencies when gases entering the three-way catalyst are near stoichiometric conditions. However, at stoichiometric conditions it may be difficult to regenerate a particulate filter. Conversely, the particulate filter may favor lean exhaust gases for regeneration, but conversion efficiency of NOx in a catalyst may degrade. The reference appears to offer little direction for overcoming these issues.
The inventors herein have developed a system for processing particulate matter of a gasoline engine having an exhaust system, comprising: a device including at least a first three-way catalyst and a particulate filter, said device is an aftertreatment device located farthest upstream in said exhaust system as compared to all other aftertreatment devices in said exhaust system; and a second three-way catalyst disposed in said exhaust system downstream of said device.
By placing a particulate filter at a location upstream of a three-way catalyst in an exhaust system, a lean exhaust mixture may be exhausted from an engine operating on gasoline while performance degradation of a downstream three-way catalyst is reduced. For example, when a particulate filter is at a temperature where soot held in the filter can be oxidized, a lean exhaust mixture can pass through the particulate filter so that at least a portion of the excess oxygen in the lean exhaust mixture oxidizes the soot held by the filter. Thus, excess oxygen in the exhaust gases can be used to oxidize the soot held by the particulate filter such that exhaust gases that flow through the downstream three-way catalyst can be near stoichiometric conditions. This allows a three-way catalyst located downstream of the particulate filter to operate at a higher efficiency as compared to a condition when a lean mixture is passed through the downstream three-way catalyst.
The present description may provide several advantages. Specifically, the description provides a method for storing and regenerating carbonaceous particulate emissions from a spark-ignited engine while maintaining net stoichiometric exhaust conditions to control tailpipe emissions. The approach may improve engine emissions by providing improved control over exhaust gases that are processed by a three-way catalyst. Further, engine air-fuel control may be simplified, especially if an oxygen sensor is placed in the exhaust system between the particulate filter and a downstream catalyst because a direct measurement of the amount of oxygen consumed by oxidizing soot is possible rather than an inference made by a model. Further still, the method regenerates a particulate filter without having to provide additional hardware, such as an air pump.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.